Molecular basis of heredity

 Griffith’s Transformation Experiment  Avery’s Transformation Experiment  Hershey-Chase Bacteriophage Experiment  Tobacco Mosaic Virus TMV Experiment  Nucleotides - composition and

The Search for the Genetic

Material of Life

 What is a gene?

 Stable source of information

 Ability to replicate accurately

 Capable of change

The Search for the Molecular Basis of Heredity

 Search for genetic material -nucleic acid or protein/DNA

or RNA?

 Griffith’s Transformation Experiment

 Avery’s Transformation Experiment

 Hershey-Chase Bacteriophage Experiment

 Tobacco Mosaic Virus (TMV) Experiment

 Nucleotides - composition and structure

 Double-helix model of DNA - Watson & Crick

 Original Source for portions of slide content:

http://mercury.bio.uaf.edu/~kevin_mccracken/genetics/lectures/chapter_02.ppt by

Kevin McCracken University of Alaska Fairbanks

Timeline of events

 1890 Weismann - substance in the cell nuclei controls

development.

 1900 Chromosomes shown to contain hereditary

information, later shown to be composed of protein & nucleic acids.

 1928 Griffith’s Transformation Experiment

 1944 Avery’s Transformation Experiment

 1953 Hershey-Chase Bacteriophage Experiment

 1953 Watson & Crick propose double-helix model of DNA

 1956 Gierer & Schramm/Fraenkel-Conrat & Singer

Demonstrate RNA is viral genetic material.

Frederick Griffith’s Transformation Experiment - 1928

“transforming principle” demonstrated with Streptococcus pneumoniae

Griffith hypothesized that the transforming agent was a “IIIS” protein

Peter J Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.

Oswald T Avery’s Transformation Experiment - 1944

Determined that “IIIS” DNA was the genetic material

responsible for Griffith’s results (not RNA).

living cell and

using the cell’s

molecular

machinery.

Structure of T2

phage

DNA & protein

Hershey-Chase Bacteriophage Experiment - 1953

Life cycle of virulent T2 phage:

1 T2 bacteriophage is

composed of DNA and

proteins:

2 Set-up two replicates:

• Label DNA with 32 P

• Label Protein with 35 S

3 Infected E coli bacteria with

two types of labeled T2

4 32 P is discovered within the

bacteria and progeny

phages, whereas 35 S is not

found within the bacteria but

released with phage ghosts.

Hershey-Chase Bacteriophage Experiment - 1953

1969: Alfred Hershey

Gierer & Schramm Tobacco Mosaic Virus (TMV) Experiment –

1956 & Fraenkel-Conrat & Singer - 1957

• Used 2 viral strains to demonstrate RNA is the genetic material of TMV

Conclusions about these early

experiments:

• Griffith 1928 & Avery 1944:

• DNA (not RNA) is transforming agent.

• Hershey-Chase 1953:

• DNA (not protein) is the genetic material.

• Gierer & Schramm 1956/Fraenkel-Conrat &

Singer 1957:

• RNA (not protein) is genetic material of

some viruses.

Nucleotide = monomers that make up DNA and RNA (Figs 2.9-10)

Three components

1 Pentose (5-carbon) sugar

DNA = deoxyribose RNA = ribose

(compare 2’ carbons)

2 Nitrogenous base

Purines Adenine Guanine

Pyrimidines Cytosine Thymine (DNA) Uracil (RNA)

3 Phosphate group attached to 5’ carbon

Nucleotides are linked by phosphodiester bonds

to form polynucleotides.

Phosphodiester bond

Covalent bond between the phosphate group (attached to 5’ carbon) of one nucleotide and the 3’ carbon of the

sugar of another nucleotide.

This bond is very strong, and for this reason DNA is

remarkably stable DNA can be boiled and even

autoclaved without degrading!

5’ and 3’

The ends of the DNA or RNA chain are not the same One end of the chain has a 5’ carbon and the other end has a 3’ carbon.

5’ end

3’ end

James D Watson & Francis H Crick - 1953

Double Helix Model of DNA

Two sources of information:

1 Base composition studies of Erwin Chargaff

• indicated double-stranded DNA consists of ~50% purines

(A,G) and ~50% pyrimidines (T, C)

• amount of A = amount of T and amount of G = amount of C

James D Watson & Francis H Crick - 1953

Double Helix Model of DNA

Two sources of information:

2 X-ray diffraction studies - Rosalind Franklin & Maurice Wilkins

Conclusion-DNA is a helical structure with

distinctive regularities, 0.34 nm & 3.4 nm.

Double Helix Model of DNA: Six main features

1 Two polynucleotide chains wound in a right-handed (clockwise)

double-helix.

2 Nucleotide chains are anti-parallel: 5’ → 3’

3’ ← 5’

3 Sugar-phosphate backbones are on the outside of the double

helix, and the bases are oriented towards the central axis.

4 Complementary base pairs from opposite strands are bound

together by weak hydrogen bonds.

A pairs with T (2 H-bonds), and G pairs with C (3 H-bonds).

3’-ATAAGGCT-3’

5 Base pairs are 0.34 nm apart One complete turn of the helix

requires 3.4 nm (10 bases/turn).

6 Sugar-phosphate backbones are not equally-spaced, resulting in

major and minor grooves.

1962: Nobel Prize in Physiology and Medicine

James D.

Watson Francis H. Crick Maurice H F. Wilkins

What about?

Rosalind Franklin

Yeast Alanine tRNA

RNA (A pairs with U and C pairs with G)

Examples:

RNA secondary structure:

single-stranded Function in

transcription (RNA processing) and translation

Organization of DNA/RNA in chromosomes

Genome = chromosome or set of chromosomes that contains all the

DNA an organism (or organelle) possesses

1 Eukaryotic chromosome structure

Chromatin - complex of DNA and chomosomal proteins ~ twice as much or more protein as DNA.

2 Eukaryotic chromosomes or chromatin found in the

nucleus of the cell.

3 Cells from different species contain varying numbers

of chromosome of different sizes and morphologies -the karyotype (e.g., pea, 2N = 14; human, 2N = 46, fruit fly, 2N= 8).